1. Field of the Invention
This invention relates to the field of infrared sensing, and more particularly concerns a method and apparatus for wide area target search and tracking.
2. Description of Related Art
Elemental infrared detectors are often used in surveillance, target seeking, and search and tracking imaging systems to sense the presence of electromagnetic radiation with wavelengths from 1-30 .mu.m. To detect infrared radiation, these elemental detectors often use temperature sensitive pyroelectric and ferroelectric materials such as triglicine sulfate and lanthanum doped lead zirconate titanate. Such crystals exhibit spontaneous electrical polarization in response to incident infrared radiation which creates a potential drop across electrodes attached to the crystals. Photoconductive materials such as lead-sulfide and mercury-cadmium-telluride may also be used in which the resistance of the material changes as a function of incident radiation. Finally, photovoltaic devices such as those fabricated from mercury-cadmium-telluride, indium antimonide, or similar materials may be used for infrared detection using a standard P/N junction where intrinsic band-to-band electron-hole excitation generates a current or voltage which is proportional to the incident radiation flux.
Arrays of such elemental detectors may be used to form thermal imaging systems or sensors. In real time thermal imaging systems such as forward looking infrared ("FLIR") imaging sensors, oscillating prism mirrors are used to scan radiation emitted by a source across a one-dimensional array of elemental detectors. When the elemental detectors are used in this manner, the temporal outputs of the detectors may be used to generate a two-dimensional representation of the image. In two-dimensional detector array imaging systems which can utilize either staring or scanning arrays, the elemental detectors produce free charge carriers or currents which may then be monitored by an appropriate readout integrated circuit such as a charge-coupled device ("CCD"). The output from the CCD can be processed by various techniques such as time delay and integration and parallel-to-serial scan conversion, with the choice depending on the system requirements of frame rate, signal-to-noise ratios, etc. It should be understood, however, that other types of readout devices may also be used.
Using such sensing devices, targets or other objects can be searched for and detected by means of the infrared radiation which that target emits. The search is typically conducted by either moving the sensor field-of-view over the projected target search area, or by having a sensor whose field of view is large enough to completely cover the target search area. In the former case, the sensor is often referred to as a gimballed or turreted sensor or FLIR. Following search and detection, the gimballed FLIR can track the target in any of several ways. Two of the most common methods for purposes of the present discussion are (1) imaging track, in which the imaging FLIR sensor line of sight is positioned on the target and maintained there or tracked in the presence of all motion, and (2) track-while-scan ("TWS") mode, in which the FLIR is moved in the search area according to a scheduled pattern and track history is maintained in a separate data processor which records, analyzes, and correlates all detections. The TWS mode is well suited to wide area search and track of multi-targets.
The chief disadvantage of using FLIR based imaging systems in the TWS mode is that such systems had to operate in a very slow search mode to prevent blurring during manual observation, or in a slow step-stare mode for manual observation of the display and for automatic target detection/recognition processing. Accordingly, imaging trackers using these approaches either were generally not capable of continuously tracking a rapidly moving object or multiple targets or required the operator to view fragments of the changing scene rather than continuously viewing the scene as it changed. While some effort was directed toward developing specific sensor designs with a specific focal plane so that they could scan a wide field-of-view more quickly, such systems did not generally incorporate a gimballed common-module FLIR as presently used in a relatively large number of applications.